CROSS-REFERENCE TO RELATED APPLICATION
The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 101 57 865.2, filed on Nov. 26, 2001, the disclosure of which is expressly incorporated by reference herein in its entirety.
The present invention relates to a programmable voltage pump having a ground option.
Programmable voltage pumps are used, for example, in memory modules, such as DRAMs, in order to produce different negative voltages for the MOS selection transistors of the memory elements (memory capacitors). In this case, the programmable voltage pumps are an integrated component of the memory modules.
In DRAM memory modules, the programmable voltage pumps are essentially used to produce two important negative voltages, the voltage (VBB) for the p-well of a selection transistor of a cell, and the voltage (VNWL) on a word line that is not selected, that is to say the gate voltage of the selection transistor.
Until now, ground potential has normally been used for a non-selected word line. In the meantime, slightly negative voltages are being introduced, in order to improve the blocking behavior of the selection transistor. This has the advantage that the leakage current through the transistor decreases as the gate voltage falls.
In contrast, the absolute value of VBB is ever more closely approaching the value of 0V. In consequence, the potential difference between the diffusion connection for the memory capacitor and the p-well is kept as low as possible, in order thus to avoid large leakage currents at this pn-junction. On the other hand, the leakage current through the transistor rises as the p-well voltage becomes less in absolute terms.
In order to set an optimum operating pointxe2x80x94that is to say the first aim is to achieve a good blocking behavior of the transistor, and secondly to achieve as low a leakage current as possible at the pn-junction of the well/diffusionxe2x80x94programmable voltage pumps are used whose output voltage can be adjusted within a predetermined voltage range. If it is found that the ground connection (0V) is the best operating point, then the voltage pump is deactivated in order to avoid consuming power unnecessarily, and the load to be driven is connected to ground.
A conventional programmable voltage pump, which is also referred to as a voltage pump, is illustrated in FIG. 1.
The illustrated programmable voltage pump has a trim input TRIM, via which a desired output voltage VNEG can be set, as well as an output OUT at which the output voltage VNEG is emitted.
When using the voltage pump 1 in a DRAM module, the output OUT may be connected either to the p-well or to the VNWL network.
The illustrated voltage pump 1 furthermore has an activation input ACTIVE, by means of which the voltage pump 1 can be activated and deactivated. If it is found in test trials that, for example, the ground connection represents the best operating point for the p-well of the selection transistor, then the voltage pump 1 is deactivated, and the appropriate network, which is connected to the output OUT, is permanently connected to ground (see optional connection 8). For this purpose, the voltage pump 1 has the option of changing the connection to the activation input ACTIVE and to the output OUT of the voltage pump 1.
The voltage pump 1 is activated by the activation input ACTIVE being connected, as illustrated, to a supply voltage VDD. In order to deactivate the voltage pump 1, the connection 7 is connected to ground. Furthermore, the output OUT of the voltage pump 1 is decoupled from the connected network (not shown), and the latter is connected to ground at the connection 8.
This procedure has a number of disadvantages: firstly, the process of changing the connections is relatively time-consuming and costly since a new lithography mask must be produced and must be introduced into the wafer production process while, secondly, the option of connecting the connected network to a negative potential is no longer available, as soon as it has been connected to ground.
A feature of the present invention provides a programmable voltage pump whose output voltage can be set in a predetermined range, including 0V.
The present invention provides a programmable voltage pump, having a trim input for setting a desired output voltage and having an output at which the output voltage is emitted, with a switch which is connected to the output and can selectively connect the output of the voltage pump to ground. In this way, it is possible to choose between a desired output voltage and ground by trimming the voltage pump or by driving the switch.
The voltage pump may provide for the deactivation of the voltage pump on selection of the ground option at the output of the voltage pump.
According to one feature of the invention, the voltage pump furthermore has a control circuit for controlling the switch as a function of the signal which is applied to the trim input.
The control circuit has means to whose input the trim signal is supplied. These means, which may be but are not limited to a logic gate, may at the same time be used for deactivation of the voltage pump. They are thus preferably connected to the activation input of the voltage pump. The logic gate may be, for example, a NAND gate.
According to one feature of the invention, the control circuit has a level shifter, which produces a specific control voltage for the output switch.
The control circuit may have a first pair of parallel-connected MOS transistors and a second pair of cross-coupled MOS transistors.
The switch which is connected to the output of the voltage pump may be in the form of an NMOS transistor, whose p-well is connected to the output voltage of the voltage pump.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawings.